Honey fungus or Armillaria is a genus of parasitic fungi that live on trees and woody shrubs. It includes about 10 species formerly lumped together as A. mellea.
Armillaria is long lived and form some of the largest living organisms in the world. The largest single organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.9 km²) and is thousands of years old. Some species of Armillaria are bioluminescent and may be responsible for the phenomena known as foxfire and perhaps will o' the wisp.
As a forest pathogen, Armillaria can be very destructive. It is responsible for the "white rot" root disease (see below) of forests and is distinguished from Tricholoma (mycorrhizal) by this parasitic nature. Its high destructiveness comes from the fact that, unlike most parasites, it doesn't need to moderate its growth in order to avoid killing its host, since it will continue to thrive on the dead material.
Bioluminescent Honey fungusAmong European rural people, especially in Gaelic, Slavic and Germanic folk cultures, the Will-o'-the-wisps are held to be mischievous spirits of the dead or other supernatural beings attempting to lead travellers astray (compare Puck). Sometimes they are believed to be the spirits of unbaptized or stillborn children, flitting between heaven and hell. Other names are Jack O' Lantern, or Joan of the Wad, Jenny Burn-tail, Kitty wi' the Whisp, or Spunkie.
Anybody seeing this phenomena might merely have been seeing, without knowing, a luminescing Barn Owl, at least in some instances. As strange as it may seem, much anecdotal evidence supports the fact that Barn Owls have a luminescence which may be due to fungal bioluminescence (Honey fungus) or some other cause.